Mr McCausland, the PSNI's Assistant Chief Constable in charge of Belfast, met his opposite number in Baghdad in July.

At the time they met, the peaceful passage of the Twelfth back home was an indication that Mr McCausland's patch is moving beyond its security nightmare. Major General Khadim Hamid Shi'wa al-Muhammadadawi, the chief of Baghdad police, is in the middle of his.

"The night before he'd had eight of his police officers murdered," Mr McCausland said this week in Belfast.

"But he was more than prepared to take on more responsibility to deal with the problems and the situations in Baghdad.

"He had a map, something similar to the map I have here, outlining Baghdad. It was clearly coloured with the areas that remain problematic for him, problematic for his officers."

The map had another familiar element: peacelines beginning to separate Sunni and Shia districts.

"This is an officer who had to live in his station because of the threat against his life. I have to admit there was a degree of respect for this officer because he's putting his life on the line. We actually joked that the chief of Belfast was there with the chief of Baghdad. I suggested tongue in cheek that he might want to come and look after Belfast and I'd go look after Baghdad," Mr McCausland said.

Most experts have been reluctant to draw parallels between Ulster and Iraq, but - with Stormont politicians advising Shi'ite and Sunni groups, and the US Marines on their way to Belfast to learn more about police liaison - it seems there are lessons from the Troubles that can usefully be applied over there.

That's what brought Mr McCausland to Baghdad. He was the only non-American on a commission set up by the US Congress to report on the state of the Iraqi security forces.

"My part in it was to bring the knowledge and expertise of the PSNI in policing a divided society and also working with the armed forces directly in terms of providing that policing service," he said.

Its report has been submitted, alongside General David Petraeus' recent account of the US troop surge. Mr McCausland has recently returned from Washington, where he testified before Congress about the findings.

The report is the product of 20 days the Independent Commission on the Security Forces of Iraq spent examining the country. During his nine days there, Mr McCausland went on patrol in Baghdad, experienced 55C heat, was flown between bases by helicopters travelling at 120 mph just 100 feet above the ground, and experienced "one or two moments where we had concern for our security".

"I have to say it was a humbling experience to see exactly what the coalition men and women are actually achieving on a day and daily basis," he said. "They were very upbeat and positive about what they're trying to do."

And he reckons Iraq can be turned into a success.

"I think the future success for Iraq actually revolves around the police service," he said. "If you can get the police service right and contributing as we would hope the police service would be able to, I think that would significantly change the role of the Iraqi army and the coalition. If you invest in the police, they can step up. The Iraqi army can step up. And the coalition can step back."

Some of the commission's recommendations can be traced directly back to Ulster - for example, the idea that police should be the primary contact for the civilian population, with the military behind them.

"The point of contact between the Iraqi public and the coalition security forces should always be led by an Iraqi police officer," Mr McCausland said. "That interface should be through an Iraqi.

"It's a little bit like what happened in Northern Ireland, where for example in parts of Belfast you had 16 soldiers around two police officers to patrol and to portray the functions of policing.

"Gradually as the environment changed we were able to reduce significantly and move right the way back to where the police became the total lead on their own. The police and the security forces in Northern Ireland created or helped to create the environment that allowed the politicians to start to heal divisions. That's in effect what Iraq should be looking at. It is happening to a certain extent, but in small steps."

He says one of the "simple, principle lessons" from Northern Ireland is "the fact that the police service must represent the community it polices".

"As you know, before the Patten reforms, the PSNI, the RUC as it was then, reflected 92% one side of the community and 8% of the other - not because of the way the police wanted to be structured but because of the terrorist situation.

"I think those principles are what's going on in Anbar province, in that they are trying to get a police service that reflects the local community."

But he says "the biggest issue that I emphasised to them was to get some form of status quo, normality established, security established so people can live in an element of safety in their homes".

Hence the peacelines: "There were mixed areas," said the assistant chief constable. "What to a certain extent has happened is that those mixed areas have become polarised. The people had to move or moved into their own communities.

"That was a re-run of Belfast in 1969. Therefore there was learning in terms of reflecting the situation we had here to the situation they currently face.

"I think you've got to be careful how you use barriers, that the barriers themselves don't become the problem. It's easy enough to put a barrier up. The hard part is when you start to make moves to take the barriers down. Thirty eight years later, I'm still sitting with barriers in Belfast and even in the last couple of weeks I've had trouble across these interfaces."

Mr McCausland says the clear downside of erecting walls is "that underlines the difference" between communities. "But when people are being killed in the numbers they're being killed in, you have to in effect give them basic security, then start addressing the reconciliation issues. If it's an alternative between having the barrier or being shot at and killed, you'll have the barrier.

"But the people behind those barriers have to in the first instance feel the barriers are there to make them feel safe. You can't just lock them behind walls and leave them, which may have happened in some areas.

He says he's hopeful that Iraq can gradually stabilise, probably through provincial governments, rather than the central government in Baghdad.

But he says success will require the coalition to stay for the foreseeable future.

"If the coalition packed up and left it would be genocide," he said. "In Baghdad and other parts of the country it would be total ethnic cleansing."

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